make sync --no-content be accepted

It's the default, but this is a step toward changing that default later..

Command/Sync.hs

@@ -47,6 +47,7 @@ import Annex.AutoMerge
 import Annex.Ssh
 import Annex.BloomFilter
 import Utility.Bloom
+import Utility.OptParse
 
 import Control.Concurrent.MVar
 import qualified Data.Map as M
@@ -67,9 +68,8 @@ data SyncOptions  = SyncOptions
 optParser :: CmdParamsDesc -> Parser SyncOptions
 optParser desc = SyncOptions
 	<$> cmdParams desc
-	<*> switch
+	<*> invertableSwitch "content" False
-		( long "content"
+		( help "also transfer file contents" 
-		<> help "also transfer file contents"
 		)
 	<*> optional (strOption
 		( long "message" <> short 'm' <> metavar "MSG"

Utility/OptParse.hs

@@ -0,0 +1,45 @@
+{- optparse-applicative additions
+ -
+ - Copyright 2015 Joey Hess <id@joeyh.name>
+ -
+ - License: BSD-2-clause
+ -}
+
+module Utility.OptParse where
+
+import Options.Applicative
+import Data.Monoid
+
+-- | A switch that can be enabled using --foo and disabled using --no-foo.
+--
+-- The option modifier is applied to only the option that is *not* enabled
+-- by default. For example:
+--
+-- > invertableSwitch "recursive" True (help "do not recurse into directories")
+-- 
+-- This example makes --recursive enabled by default, so 
+-- the help is shown only for --no-recursive.
+invertableSwitch 
+	:: String -- ^ long option
+	-> Bool -- ^ is switch enabled by default?
+	-> Mod FlagFields Bool -- ^ option modifier
+	-> Parser Bool
+invertableSwitch longopt defv optmod = invertableSwitch' longopt defv
+	(if defv then mempty else optmod)
+	(if defv then optmod else mempty)
+
+-- | Allows providing option modifiers for both --foo and --no-foo.
+invertableSwitch'
+	:: String -- ^ long option (eg "foo")
+	-> Bool -- ^ is switch enabled by default?
+	-> Mod FlagFields Bool -- ^ option modifier for --foo
+	-> Mod FlagFields Bool -- ^ option modifier for --no-foo
+	-> Parser Bool
+invertableSwitch' longopt defv enmod dismod = collapse <$> many
+	( flag' True (enmod <> long longopt)
+	<|> flag' False (dismod <> long nolongopt)
+	)
+  where
+	nolongopt = "no-" ++ longopt
+	collapse [] = defv
+	collapse l = last l